Measuring devices which are implemented for progressive tracking of a target point and a coordinate position determination of this point can generally, in particular in conjunction with industrial surveying, be summarized under the term laser trackers. A target point can be represented in this case by a retroreflective unit (for example, a cube prism), which is targeted using an optical measurement beam of the measuring device, in particular a laser beam. The laser beam is reflected in parallel back to the measuring device, wherein the reflected beam is detected using a detection unit of the device. An emission or reception direction of the beam is ascertained in this case, for example, by means of sensors for angle measurement, which are associated with a deflection mirror or a targeting unit of the system. In addition, a distance from the measuring device to the target point is ascertained with the detection of the beam, for example, by means of runtime or phase difference measurement.
Laser trackers according to the prior art can additionally be embodied having an optical image detection unit having a two-dimensional, light-sensitive array, for example, a CCD or CID camera or a camera based on a CMOS array, or having a pixel array sensor and having an image processing unit. The laser tracker and the camera can be installed one on top of another in this case in particular in such a manner that the positions thereof in relation to one another are not variable. The camera is, for example, rotatable together with the laser tracker about its essentially perpendicular axis, but is pivotable up-and-down independently of the laser tracker and is therefore arranged separately from the optic of the laser beam in particular. Furthermore, the camera—for example, as a function of the respective application—can be embodied as pivotable about only one axis. In alternative embodiments, the camera can be installed in an integrated construction together with the laser optic in a shared housing.
With the detection and analysis of an image—by means of an image detection and image processing unit—of a so-called measuring aid instrument having markings, the relative locations of which to one another are known, an orientation of the instrument and of an object (for example, a probe), which is arranged on the measuring aid instrument, in space can be concluded. Together with the determined spatial position of the target point, furthermore the position and orientation of the object in space can be precisely determined absolutely and/or in relation to the laser tracker.
Such measuring aid instruments can be embodied by so-called scanning tools, which are positioned having the contact point thereof on a point of the target object. The scanning tool has markings, for example, light spots, and a reflector, which represents a target point on the scanning tool and can be targeted using the laser beam of the tracker, wherein the positions of the markings and of the reflector in relation to the contact point of the scanning tool are precisely known. The measuring aid instrument can also be, in a way known to a person skilled in the art, a handheld scanner equipped for distance measurement, for example, for contactless surface surveying, wherein the direction and position of the scanner measurement beam used for the distance measurement are precisely known in relation to the light spots and reflectors which are arranged on the scanner. Such a scanner is described, for example, in EP 0 553 266.
In addition, in modern tracker systems, a deviation of the received measurement beam from a zero position is ascertained on a sensor—increasingly as a standard feature. By means of this measurable deviation, a position difference between the center of a retroreflector and the point of incidence of the laser beam on the reflector can be determined and the alignment of the laser beam can be corrected or tracked as a function of this deviation such that the deviation on the sensor is decreased, in particular is “zero”, and therefore the beam is aligned in the direction of the reflector center. By way of the tracking of the laser beam alignment, progressive target tracking (tracking) of the target point can be performed and the distance and position of the target point can be progressively determined in relation to the measuring device. The tracking can be implemented in this case by means of an alignment change of the deflection mirror, which is movable by a motor, provided for deflecting the laser beam and/or by pivoting the targeting unit, which has the beam-guiding laser optic.
The described target tracking must be preceded by locking of the laser beam on the reflector. For this purpose, a detection unit having a position-sensitive sensor and having a comparatively large field of vision can additionally be arranged on the tracker. In addition, in devices of this type, additional illumination means are integrated, using which the target or the reflector is illuminated, in particular using a defined wavelength differing from the wavelength of the distance measuring means. The sensor can be implemented in this context as sensitive to a range around this specific wavelength, for example, to reduce or entirely prevent external light influences. By means of the illumination means, the target can be illuminated and, using the camera, an image of the target having an illuminated reflector can be detected. By way of the imaging of the specific (wavelength-specific) reflection on the sensor, the reflection position in the image can be resolved and therefore an angle in relation to the detection direction of the camera and a direction to the target or reflector can be determined. An embodiment of a laser tracker having such a target search unit is known, for example, from WO 2010/148525 A1. In dependence on the direction information thus derivable, the alignment of the measurement laser beam can be changed such that a distance between the laser beam and the reflector onto which the laser beam is to be locked is decreased.
Laser trackers of the prior art have at least one distance meter for distance measurement, wherein it can be implemented as an interferometer, for example. Since such distance measuring units can only measure relative distance changes, so-called absolute distance meters are installed in addition to interferometers in current laser trackers. For example, such a combination of measuring means for distance determination is known by way of the product AT901 of Leica Geosystems AG. The interferometers used in this context for distance measurement primarily use gas lasers—as a result of the long coherence length and the measurement range thus made possible—as light sources, in particular HeNe gas lasers. The coherence length of the HeNe laser can be several hundred meters, so that the ranges required in industrial metrology can be achieved using relatively simple interferometer constructions. A combination of an absolute distance meter and an interferometer for distance determination using a HeNe laser is known, for example, from WO 2007/079600 A1.
By way of the use of such an interferometer for distance determination or determination of the distance change in a laser tracker, a very high measurement precision can be implemented as a result of the interferometric measuring method thus usable.
However, this advantageous measurement precision is disadvantageously opposed by the robustness and reliability of the interferometer measurement to be executed. To execute a correct measurement of the distance change, in particular a progressively correct measurement during tracking of a target, by means of the interferometer, detection, which is continuous during the measurement, and correct readout of the interferometer pulses generated by interference effects (intensity maxima and minima) must be ensured. The determination of the distance change is dependent in this case on the number of the detected interferometer pulses. Uninterrupted reception and recognition of the interferometer pulses can be interfered with, in particular in the event of a large distance between the interferometer and the target, since the measurement radiation reflected from the target in this case is detected with comparatively low intensity and the available sensitivity of the interferometer detector is not sufficient to clearly detect the pulses. Due to a loss thus caused of one or more interferometer pulses during the detection, the determination of the distance change dependent thereon can be erroneous. An erroneous detection of the pulses (one or more pulses are not counted) can additionally be caused by a rapid offset of the reflective target and an intensity fluctuation or intensity reduction thus generated at the interferometer detector. This can occur in particular during tracking of a target, if the movement of the reflector occurs more rapidly than a servo-controlled tracking of the laser radiation to the target can be carried out. Since the determinable distance change is dependent on the number of recognized pulses, an erroneous distance measured value can thus be generated.
A further disadvantageous aspect in this context is that in the case of an above-described erroneous measurement, a measured value can be generated, but a user of the system does not recognize this measurement error or cannot recognize it, as a result of the achievable measurement resolution, and assumes the generated measured value to be correct. Individual incorrect measurements can accumulate due to repeated such non-consideration of the error and a resulting (total) measurement error can therefore be enlarged.